The Electronic Industry Association (EIA) 485 is an electrical standard, specifying balanced (differential) drivers and receivers for digital data transfer between interfaces of Data Transmission Equipment (DTE), such as a computer terminal, and Data Communication Equipment (DCE), such as a modem. Single ended data transmission suffers from susceptibility to Electromagnetic interference (EMI) noise coupling and generates EMI on the fast rise and fall edges of the data. Differential transmission techniques are not as susceptible to the noise since it is added to both lines in a common mode sense and thus the receiver does not see the longitudinal noise. Differential techniques simply do not generate as much EMI on the transition edges as long as the propagation skew is tolerable. It is very difficult, however, to achieve tolerable skew and to achieve acceptable slew rate control at the same time.
Conventional slew rate techniques using current steering architecture suffer from the unequal propagation delays of the controlled driver which makes the skew control cumbersome. These propagation delays can be directly attributed to the propagation delays in the transfer of signal from the input port to the output port of the current steering element, which may be, for example, a transconductor.
It is accordingly an object of the invention to provide a technique of achieving both slew control and skew in differential driver circuits.
Other objects and benefits of the invention will be apparent to those of ordinary skill in the art having the benefit of the description and drawings following herein.